linux-kernel - Re: [PATCH 0/2] powerpc/kvm: Enable running guests on RT Linux

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Message-ID: <54E74B58.90706@redhat.com>
Date:	Fri, 20 Feb 2015 15:57:28 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Alexander Graf <agraf@...e.de>,
	Bogdan Purcareata <bogdan.purcareata@...escale.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-rt-users@...r.kernel.org
CC:	linux-kernel@...r.kernel.org, scottwood@...escale.com,
	mihai.caraman@...escale.com, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/2] powerpc/kvm: Enable running guests on RT Linux



On 20/02/2015 15:54, Sebastian Andrzej Siewior wrote:
> Usually you see "scheduling while atomic" on -RT and convert them to
> raw locks if it is appropriate.
> 
> Bogdan wrote in 2/2 that he needs to limit the number of CPUs in oder
> not cause a DoS and large latencies in the host. I haven't seen an
> answer to my why question. Because if the conversation leads to
> large latencies in the host then it does not look right.
> 
> Each host PIC has a rawlock and does mostly just mask/unmask and the
> raw lock makes sure the value written is not mixed up due to
> preemption.
> This hardly increase latencies because the "locked" path is very short.
> If this conversation leads to higher latencies then the locked path is
> too long and hardly suitable to become a rawlock.

Yes, but large latencies just mean the code has to be rewritten (x86
doesn't anymore do event injection in an atomic regions for example).
Until it is, using raw_spin_lock is correct.

Paolo
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